Small air-cooled internal combustion engines, those uesd, for example, for lawn mowers and other portable equipment, typically have engine heads of an aluminum alloy, with an exhaust muffler assembly, typically of steel, attached to the head. During operation of the engine the muffler becomes very hot from the heat of the exhaust gasses and the head. After the engine is shut off and the engine cools, heat flows back from the steel muffler assembly into the head. This heat flowing back into the aluminum head can cause localized hot spots, which in turn can lead to damage of the head.
This problem has been traditionally solved by use of an asbestos gasket placed between the muffler and the head. Asbestos gaskets have been successful in preventing head damage from heat feed-back from the muffler, but because of environmental problems associated with asbestos, there is a need in the industry to provide an inexpensive non-asbestos gasket in this application.
In order for a material to function successfully as a gasket between the aluminum head and the muffler, it must sufficiently inhibit heat feed-back from the muffler to prevent damage to the head. In addition, the material must be sufficiently resilient to provide a seal between the head and muffler and provide a minimum of leakage of exhaust gasses. The gasket should also be of sufficient strength to maintain its integrity during handling, installtion. A resistance to degradation at high temperatures is also important, due to the high temperatures of the exhaust gasses, head, and the muffler, during operation of the engine.
Gaskets for engine heads, exhaust mufflers, and the like, on internal combustion engines have been made as composites of flexible graphite and a reinforcing material, such as a steel mesh or a sheet of steel or a polymeric material. Examples of these gaskets are disclosed in U.S. Pat. Nos. 4,463,959, 4,516,782, 4,234,638, and 4,519,619. While these gaskets provide a seal against exhaust gasses, and have high temperature resistance, they are not suitable as gaskets for use between aluminum heads and steel mufflers in air cooled engines where heat flow back is a problem. The primary materials used for these gaskets, i.e. metals and flexible graphite, are thermal conductors, and provide a heat conduction path between the head and the muffler. Therefore, these gaskets are not suitable in inhibiting the flow of heat from the muffler to the head.
In addition, many of these gaskets incorporate polymeric materials which do not have sufficient thermal resistance to withstand the high temperatures encountered at the exhaust muffler in air-cooled engines. At operating temperatures found in mufflers, these materials would deteriorate, and the gasket would not provide an adequate seal to exhaust gasses.